Use of zinc ferrite-based solids in a process for deep desulfurization of hydrocarbon fractions

ABSTRACT

The invention relates to a process for desulfurization of a non-oxidized hydrocarbon feedstock, comprising organic sulfur compounds, by adsorption, especially chemisorption of sulfur on a composition in bulk form consisting essentially of 70% by weight of zinc ferrite and optionally iron oxides or zinc oxides. The process is performed in the presence of hydrogen at a temperature of between 200° C. and 450° C.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to a concurrently filed application entitled “Use of Solids Based on Zinc Ferrite In a Process For Deep Desulphurizing Oxygen-Containing Feeds” by Arnaud Baudot et al., attorney docket number PET-2552, based on French Priority application Ser. No. 08/05.624. Same related application has been incorporated by reference herein.

The future specifications on automobile fuels call for a great reduction of the sulfur content in the fuels. European legislation defines the specifications of gas-oil fuels that have had 50 ppm of sulfur since 2005 and will have 10 ppm of sulfur in 2009. The evolution of the specifications of the sulfur content in the fuels requires the improvement of existing catalytic hydrotreatment processes or the development of new processes for deep desulfurization of gas oils.

Among the new methods for desulfurization of gas oils, the processes for purification by adsorption of sulfur compounds on a selective adsorbent offer an advantageous alternative to the standard hydrodesulfurization processes. This invention relates to a process for deep desulfurization on a zinc ferrite.

It is recalled that a standard hydrodesulfurization unit transforms the essential part of the sulfur compounds that are contained in a distillate into H₂S under temperature conditions that are close to 450° C., and pressures between 20 and 60 bar. However, a portion of the sulfur compounds is called “refractory” in hydrodesulfurization because their transformation into H₂S requires clearly higher pressure and temperature conditions. These refractory molecules are a part of the family of alkylated dibenzothiophenic compounds. The content of post-hydrodesulfurization refractory compounds to be treated depends on several parameters, including pressure level and hydrodesulfurization unit temperature, the quantities of catalysts involved, and the hourly volumetric flow rate that is implemented. The content of refractory compounds in the effluent of a hydrodesulfurization unit is in general greater than 15 ppmS and in general less than 500 ppmS, and even 350 ppmS, and even 50 ppmS.

During purification by adsorption, as considered in this invention, the refractory compounds will react selectively in the presence of hydrogen with the solid, releasing the organic molecule without the sulfur atom, which remains trapped in the adsorbent in a sulfide form. This adsorption with modification of molecules and adsorbent is called a reactive chemisorption. The fact that the sulfur is directly trapped on the oxide makes it possible to shift the balance of the reaction very greatly and is particularly suitable for purification treatment. As a result, contrary to the conventional hydrodesulfurization processes with which it is necessary to increase the partial pressure of hydrogen to increase the release of sulfur in H₂S form of the organic sulfur compounds, the purification by adsorption does not require the use of a high hydrogen pressure. This leads to a reduction of the investment and operating costs, in particular on the level of hydrogen compressors, in the equipment concerned. In addition, whereby the sulfur is trapped in the oxide, the hydrogen that exits from the reactor where the desulfurization stage takes place does not contain sulfur (in particular in H₂S form). It is therefore unnecessary to treat the hydrogen again so as to remove its sulfur from it.

PRIOR ART

The patent application US 2006/0191821A1 describes a process for desulfurization on a zinc oxide that is optionally promoted by an iron or copper oxide that is supported on a porous substrate based on silica and/or alumina.

The patent application WO 2007/021084 describes mixed zinc and aluminum oxides used in desulfurization processes. These solids are reduced before use.

Document WO 01/70393 A1 by Khare describes the binding of zinc ferrite particles with a binder such as alumina. The resultant solid reduced zinc ferrite-alumina particles can be used to remove sulfur from a cracked gasoline or diesel fuel stream.

The U.S. Pat. No. 4,985,074 describes the use of coked feedstocks and naphtha feedstocks of solids based on copper-zinc oxides or copper-zinc-alumina oxides that are obtained by co-precipitation. A reduction of the solid under hydrogen before the desulfurization stage is carried out.

SUMMARY DESCRIPTION OF THE INVENTION

The invention relates to a process for desulfurization of a non-oxidized hydrocarbon feedstock, comprising organic sulfur compounds, by collection of sulfur on a bulk compound that consists essentially of more than 70% by weight of zinc ferrite and optionally iron oxides or zinc oxides. The process is performed in the presence of hydrogen at a temperature of between 200° C. and 450° C.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for desulfurization of a preferably non-oxidized hydrocarbon feedstock, preferably a gasoline, kerosene or diesel fraction, preferably a diesel fraction, comprising organic sulfur compounds, preferably cyclic sulfur compounds, by collection of sulfur on a bulk compound that consists essentially of more than 70% by weight of zinc ferrite, preferably more than 80% by weight of zinc ferrite, more preferably more than 98% by weight of zinc ferrite, even more preferably more than 99.5% by weight of zinc ferrite and optionally iron oxides and/or zinc oxides, whereby said process is performed in the presence of hydrogen at a temperature of between 200° C. and 450° C.

The pressure is generally between 0.2 and 3.5 MPa, preferably between 0.5 and 3 MPa, and even preferably between 0.5 and 1.5 MPa. The hourly volumetric flow rate of the feedstock to be treated is generally between 0.1 h⁻¹ and 10 h⁻¹, preferably between 0.5 h⁻¹ and 5 h⁻¹. The hourly volumetric flow rate or VVH of treated liquid feedstock is defined as the volumetric flow rate of treated liquid feedstock on the solid bulk compound volume. The hydrogen/hydrocarbon feedstock volumetric ratio is generally between 5 and 400, preferably between 50 and 300. The hydrogen flow rates and that of liquid feedstock to be treated are taken up under normal conditions.

The zinc ferrite-type mixed oxide according to the invention is very active even under mild operating conditions relative to the conventional hydrodesulfurization conditions and to “high-pressure” hydrodesulfurization conditions designed to reach sulfur contents in the fuels that are set by the most recent standards promulgated in developed countries. While operating at hydrogen pressures of between 0.5 and 3 MPa, it makes it possible to reduce the refractory sulfur compounds to contents that are similar to that which is reached by the high-pressure hydrodesulfurization units that operate at 6 MPa and more.

The process according to the invention makes it possible to produce a desulfurized hydrocarbon fraction with contents that are less than or equal to 10 ppm of sulfur, preferably less than 5 ppm of sulfur, and even preferably less than 1 ppm of sulfur. This process is particularly effective for eliminating the refractory sulfur compounds with conventional hydrodesulfurization treatments such as dimethyldibenzothiophene in gas-oil fractions.

The process according to the invention constitutes a simple alternative to the “high-pressure” hydrodesulfurization process that makes it possible to revamp at lower cost “low-pressure” conventional hydrodesulfurization units to reach the sulfur contents set by the most recent standards.

The iron- and zinc-based mixed oxide of this invention can also be used in multicycling by making sulfur adsorption stages and solid regeneration stages alternate by oxidizing means.

In general, increasing the temperature makes it possible to desulfurize the most refractory sulfur compounds. In the case of sulfur compounds such as thiophene, the zinc ferrite is active from 150° C., whereas in the case of benzothiophenic compounds, it is active from 200° C., and in the cases of particularly refractory sulfur compounds of alkylated dibenzothiophene compounds, the zinc ferrite is active from 300° C.

An adsorption cycle that uses standard adsorbents consists of a series of recurring stages:

-   -   Stage 1: Activation phase of the solid. This pre-reduction stage         is difficult to operate industrially in a stationary bed because         of the exothermicity of this reaction.     -   Stage 2: Adsorption phase: The evaporated feedstock is brought         into contact with the hydrogen on the adsorbent. Once the solid         is saturated with sulfur or once the sulfur specifications of         the effluent are no longer observed, the solid is to be         regenerated.     -   Stage 3: Draining and immobilization phase of the adsorption         column. This involves a stripping phase with an inert gas to         eliminate the partially converted hydrocarbons and the gases         that are present in the pore volume. The purpose is actually to         prevent any mixing of hydrogen that is used in the adsorption         phase with the oxygen that is used in the regeneration phase.     -   Stage 4: Stage for regenerating the adsorbent. The regeneration         is carried out in general by a mild oxidation by using, for         example, greatly diluted oxygen to control the combustion.     -   Stage 5: Draining phase of the adsorbent bed with an inert gas         for eliminating any trace of residual oxygen in the pore volume.

Over the entire cycle, Stage No. 2 constitutes the single productive stage in the desulfurization process whereas the other stages, although necessary, constitute a non-productive down time in the desulfurization process. As a result, the operator of this type of separation operation should aim at maximizing the proportion of the duration of the phase No. 2 relative to the total cycle time.

This invention has a certain technological advantage since the implementation of the zinc ferrite does not require an activation stage. As a result, the “adsorption phase duration/total cycle duration” ratio is greatly improved. The productivity of the desulfurization operation by zinc ferrite according to the invention is still greater than that of the standard processes.

Another advantage resides in the fact that whereby the activation stage is exothermic, it is necessary that it regulate its behavior very precisely, without which irreversible mechanisms of structural modifications of the adsorbent, induced by the temperature (sintering, phase shift, . . . ) can take place. In the absence of this stage, the behavior of the zinc ferrite according to the invention for the deep desulfurization of hydrocarbon feedstocks is easier than that of standard adsorbents, such as those claimed in the prior art.

The process for preparation of a zinc ferrite-type mixed oxide generally comprises:

-   -   A stage for co-precipitation of a mixture of precursor salts of         zinc II and iron III, in the presence of a base, at a pH of         between 6.1 and 6.9 and at a temperature of between 30° C. and         50° C.     -   A filtration stage of the precipitate that is obtained     -   A drying stage for a duration of between 12 and 24 hours at a         temperature of between 125° C. and 175° C.     -   A calcination stage in the presence of oxygen at a temperature         of between 600° C. and 700° C. for a duration of between 1 hour         and 3 hours.

The zinc ferrite of formula ZnFe₂O₄ generally has a franklinite-type crystalline structure.

The size of the zinc ferrite crystallites is generally between 20 and 5000 Å, and preferably between 100 and 1000 Å.

The specific surface area of the zinc ferrite is generally between 2 and 100 m²/g. The zinc ferrite is active even for specific surface areas that are less than 10 m²/g. The zinc ferrite can be used in the form of free flowing powder, balls or extrudates. The zinc ferrite is preferably run in a fixed bed, but it is also possible to run it in a circulating bed.

The zinc ferrite-type mixed oxide is generally obtained by a co-precipitation followed by a calcination. The preparation process does not require an intermediate stage of impregnation of a second phase that acts as a promoter. The zinc ferrite mixed oxide is active even with specific surface areas that are less than 10 m²/g. The synthesis according to the invention of an active zinc ferrite-based mass does not require a sophisticated protocol whose purpose is to develop a large specific surface area that is necessary for a high reactivity of this solid with regard to sulfur molecules. The process for preparation does not require a reduction stage so as to make the oxide active nor a promoter dispersion stage (for example, an iron oxide or copper oxide) on the oxide. Such a reduction stage, for example with hydrogen, is generally difficult to operate industrially in a fixed bed due to the exothermicity of the reaction.

EXAMPLES Example No. 1 (According to the Invention): Preparation of a Zinc Ferrite

A foot of water is introduced into a double-envelope borosilicate glass reactor and then heated at 40° C. under a stifling power of about 150 W/m³ released by a rotor with axial flow rate of the propeller type with blades.

The precursors are aqueous solutions of zinc (II) nitrate and iron (III) nitrate. The bulk concentrations of zinc and iron are respectively 13 g/l and 22.5 g/l.

The base is an aqueous ammonia solution. The bulk concentration of ammonia is 225 g/l.

The precursors and the base are introduced into the reactor via a pumping system that makes it possible to regulate the introduction flow rates and the duration of the synthesis. The management of the pH is ensured by the flow rate of the basic pump: it is kept constant at 6.5±0.2 throughout the co-precipitation.

During the reaction, a stirring power of about 75 W/m³ is applied in the reaction medium, and a temperature of 40° C.±2° C. is maintained in the reactor via a thermostated bath.

The precipitate is hot-filtered in a Büchner flask. The wet cake that is obtained after 45 minutes of filtration is dried in the oven for 18 hours at a temperature of 150° C.

The solid that is obtained is then calcined in the presence of molecular oxygen at a temperature of 650° C. for 2 hours.

The solid that is obtained is characterized by X-ray diffraction via a Bragg-Brentano-type powder diffractometer in a θ-θ configuration. The recording conditions are as follows: an anticathode voltage adjusted to 35 kV, the intensity in the anticathode filament set at 35 mA, the sampling span equal to 0.05°2θ, the counting time by span set at 5 s, and an angular domain that ranges from 2 to 72°2θ. On the experimental diffractogram that is obtained on our solid, the position of the lines is similar to that of a known crystallographic structure that is listed in the database “Powder Diffraction File” corresponding to the franklinite ZnFe₂O₄ (PDF No. 00-022-1012).

The positions of the most intense experimental lines are as follows for our solid: 29.93°2θ-35.27°2θ-56.61°2θ-62.15°2θ. For the franklinite, they are 29.92°2θ-35.26°2θ-56.63°2θ-62.21°2θ.

As for the mesh parameter (a=b=c in the case of a cubic system), it is identical, i.e., equal to 8.44 Å. For our solid, the mean size of zinc ferrite crystallites is 410±40 Å.

A semi-quantitative analysis by X fluorescence has also been carried out on the synthesized solid. The contents that are obtained after correction of a fire loss carried out at 550° C., 4 h (PAF=0.3%) leads to the following contents: % by weight of Fe=42.48±0.74% and % by weight of Zn=23.18±0.78%.

Finally, the specific surface area developed by the solid has been estimated by a low-temperature nitrogen volumetric analysis according to the ASTM Standard D 3663-84 or NFX 11-621: it is equal to 6±1 m²/g.

Example No. 2 (According to the Invention): Desulfurization Test by Chemisorption on Zinc Ferrite with a Model Diesel-Type Feedstock that Contains a Refractory Sulfur Compound

The second example describes the use of the solid in a fixed-bed-type reactor. 12 grams of solid in powder form is introduced into a column with an inside diameter of 1 cm and a useful volume of 9 cm³. The column is placed in a ventilated oven that operates at 380° C. The regulation of the temperature is an external regulation with measurement of the column wall temperature, which makes it possible to work without a thermowell and to eliminate any preferred path in the column.

The temperature of the output effluent is maintained and sampled by means of a sampling loop for an analytical online follow-up. The compounds are analyzed by gas-phase chromatography equipped with an FID detection and a PFPD analyzer.

A model liquid feedstock that consists of dodecane and that contains 50 ppmS by mass of 2,4 dimethyldibenzothiophene is fed with a VVH of 4 h⁻¹ by means of a Gilson syringe pump, and then evaporated in the presence of hydrogen by means of a dedicated device before being injected into the reactor. The pressure in the reactor is 12 bar, and the hydrogen/hydrocarbon molar ratio at the inlet of the reactor is equal to 200.

Upon the contact of the feedstock with the solid, an almost immediate drop in the sulfur content in the effluent of the reactor to values that are lower than 5 ppm by mass is observed. The sulfur profile based on time at the outlet of the reactor is thus kept constant and less than 5 ppm before the piercing (breakthrough) phenomenon that corresponds to a rise in the sulfur concentration until reaching the sulfur concentration value at the inlet when the adsorbent is completely saturated with sulfur. Before piercing, no trace of sulfur can be detected by the PFPD detector of the gas phase chromatograph.

It is possible to distinguish two parameters that represent the performance levels of the solid that is based on zinc ferrite:

-   -   The dynamic capacity that corresponds to the sulfur content that         is trapped in the adsorbent just before the piercing. Under the         operating conditions that are used, the dynamic sulfur capacity         of the adsorbent based on zinc ferrite is 6.8% by mass.     -   The saturation capacity that corresponds to the maximum sulfur         capacity of the adsorbent that is measured after saturation.         Under the operating conditions that are used, the sulfur         capacity with saturation of the zinc ferrite-based adsorbent is         13.4% by mass.

Example No. 3 (According to the Invention): Desulfurization Tests by Chemisorption on Zinc Ferrite of a Polycyclic Sulfur Compound in Vapor Phase

The adsorption reaction of pure dibenzothiophene (DBT) has been studied by pressurized vapor phase in an open reactor with a flushed bed. The test unit that is used consists of three parts:

-   -   The device for introducing reagents,     -   The reaction device,     -   The analysis device.

The device for introducing reagents makes it possible to use hydrogen and DBT. The H₂ flow rate is regulated by means of a bulk flowmeter of the Brooks trademark (0-100 cm³·min⁻¹).

The vapor pressure of the DBT is regulated using a saturator-condenser system. The hydrogen bubbles in the DBT that is contained in the saturator and is kept at the temperature T_(S). The gas mixture then passes through a coil (the condenser) that is kept at the temperature T_(C), less then T_(S), which allows a recondensation of the DBT. Using this system, the vapor pressure of the DBT is absolutely stable within the remainder of the device. The systems for reaction and analysis should be kept at a temperature that is higher than T_(C). The H₂-DBT flow is then introduced into the reactor that contains the solid.

The reaction device consists of a Pyrex glass tube that has a 12 mm diameter with a frit that is inserted into a stainless steel reactor with a slightly larger diameter. A graphite Teflon seal ensures the air-tightness and can withstand temperatures of 300° C. The reaction temperature is identified at the frit with a thermowell that allows the passage of a thermocouple. After the reactor, a needle valve makes it possible to regulate the pressure in the device and to ensure the stress relief at atmospheric pressure.

After the reaction and the stress relief at atmospheric pressure, the gas mixture is analyzed in a chromatogram (Hewlett Packard 5890 Series II) with detection by flame ionization (FID). These analyses make it possible for us to obtain breakthrough curves that represent the evolution of the DBT concentration at the outlet based on time.

280 mg of solid (in the form of sieved powder between 80 and 125 μm) that is manufactured according to the description of Example No. 1 is introduced into the reactor and heated under N₂ at 350° C. with a temperature slope of 10° C./minute. When the temperature is stable, the H₂/DBT gas mixture is sent to the solid with a flow rate of 70 ml/minute and a pressure of 7.10⁵ Pa (partial pressure of DBT=195 Pa).

When the DBT concentration at the outlet of the reactor is stable (or equal to that of the inlet), a bypass of the reactor is carried out. Then, the unit is flushed under H₂ so as to eliminate the DBT that is present in the unit. The unit and the reactor are then flushed under N₂ for about 1 hour, and then the heating of the reactor is stopped.

Under these conditions, it has been shown that the DBT content was zero in hydrogen at the outlet of the reactor before piercing and that the dynamic sulfur capacity of the zinc ferrite was 8.8% by mass. Its sulfur capacity at saturation is equal to 15.3% by mass.

Example No. 4 (According to the Invention): Desulfurization by Chemisorption on Zinc Ferrite of a Model “Heavy Gasoline”-Type Feedstock

The device and the quantity of zinc ferrite used in this example are similar in all respects to what is described in Example No. 2.

A model liquid feedstock that consists of 80% decane and 20% toluene containing 50 ppmS by mass of 3-methylbenzothiophene is fed with a VVH of 4 h⁻¹ by means of a Gilson syringe pump, then it is evaporated in the presence of hydrogen by means of a dedicated device and then injected into the reactor. The pressure in the reactor is 15 bar, and the hydrogen/hydrocarbon molar ratio at the inlet of the reactor is 200.

Upon the contact of the feedstock with the adsorbent, an almost immediate drop in the sulfur content in the effluent of the reactor to values of less than 5 ppm by mass is observed. The sulfur profile based on time at the outlet of the reactor is thus kept constant and less than 5 ppm before the piercing phenomenon. Before piercing, no trace of sulfur was detected by the PFPD detector of the gas phase chromatograph. Under the operating conditions that are described in this patent, the dynamic sulfur capacity of the adsorbent with a zinc ferrite base is 9.8% by mass, and its sulfur capacity at saturation is 14.8% by mass.

Example No. 5 (Comparison Test with a Zinc Ferrite-Based Oxide Supported on an Alumina)

Starting from zinc ferrite and boehmite powders, a solid is prepared by mixing-extrusion. The percentages by mass of zinc, iron and aluminum are respectively 5.4%, 9.3% and 21.2%. The oxide powders are mixed in a mixer in the presence of acidified water. After mixing for 30 minutes, at 25 rpm, the paste that is obtained is extruded on a piston extruder with a shift of the piston of 10 mm/minute, through a die with a 3 mm diameter. The rods are finally dried for one night at 80° C. in the oven and calcined at 650° C. for 2 hours in air. After this calcination stage, the solid was analyzed by DRX and scanning electronic microscopy, and it showed the presence of phases that are for the most part zinc ferrite and alumina.

The solid has a specific surface area of 200 m²/g, whereby the alumina greatly contributes to providing this property.

The device and the quantity of solid used in this example are similar in all respects to what is described in the Example Nos. 2 and 4.

Under these conditions, it was shown that the piercing (breakthrough) time is faster than in the case of zinc ferrite alone and that the dynamic sulfur capacity of the adsorbent drops to 1.5% by mass for a total capacity of 1.8% by mass. If the zinc ferrite that is deposited on alumina is compared, it appears very clearly that its sulfur capacity, either dynamic or with saturation, is extremely reduced relative to the zinc ferrite alone. The very large specific surface area offered by this composite relative to the zinc ferrite alone (specific surface area of the zinc ferrite/alumina/specific surface area of the zinc ferrite alone>20) therefore does not make it possible to compensate for the dilution effect of the zinc ferrite by alumina.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application Ser. No. 08/05.623, filed Oct. 10, 2008 are incorporated by reference herein.

In claim 1, the term “consisting essentially of” is meant to preclude additional components which would materially lower the sulfur capacity of the bulk adsorbent and in particular is meant to preclude the presence of binders as mentioned in WO 01/70393 by Khare. 

1. A process for desulfurization of a hydrocarbon feedstock comprising sulfur compounds, comprising subjecting said feedstock, in the presence of hydrogen at a temperature of between 200° C. and 450° C., to chemisorption on a bulk adsorption agent consisting essentially of zinc ferrite.
 2. A process according to claim 1, conducted under a pressure of between 0.2 and 3.5 MPa.
 3. A process according to claim 1, conducted with an hourly volumetric flow rate of the feedstock to be treated of between 0.1 h⁻¹ and 10 h⁻¹.
 4. A process according to claim 1, conducted with a hydrogen/hydrocarbon feedstock volumetric ratio of between 5 and
 400. 5. A process according to claim 1, in which the hydrocarbon feedstock is a gasoline, kerosene or diesel fraction.
 6. A process according to claim 1, in which the hydrocarbon feedstock is a diesel fraction.
 7. A process according to claim 1, in which the hydrocarbon feedstock comprises cyclic sulfur compounds.
 8. A process according to claim 1, in which the bulk adsorption agent comprises more than 80% by weight of zinc ferrite.
 9. A process according to claim 1, in which the bulk adsorption agent comprises more than 98% by weight of zinc ferrite.
 10. A process according to claim 2, in which the pressure is between 0.5 and 3 MPa.
 11. A process according to claim 1, further consisting essentially of zinc oxide and/or iron oxide.
 12. A process according to claim 1, wherein the hydrocarbon feedstock is a non-oxidized feedstock.
 13. A process according to claim 11, wherein the hydrocarbon feedstock is a non-oxidized feedstock.
 14. A process according to claim 1, wherein the bulk adsorption agent is a free flowing mass of powder having a particle size of between 80 and 125 microns wherein the zinc ferrite consists essentially of crystallites having a size between 20 and 5000° Å.
 15. A process according to claim 14, wherein the size of the crystallites is between 100 and 1000 Å.
 16. A process according to claim 15, in which the bulk adsorption agent comprises more than 98% by weight of zinc ferrite. 